Introduction

The syntax and semantics of the regular expressions
supported by PCRE are described below. Regular expressions are
also described in the Perl documentation and in a number of
other books, some of which have copious examples. Jeffrey
Friedl's "Mastering Regular Expressions", published by
O'Reilly (ISBN 1-56592-257-3), covers them in great detail.
The description here is intended as reference documentation.

A regular expression is a pattern that is matched against a
subject string from left to right. Most characters stand for
themselves in a pattern, and match the corresponding
characters in the subject. As a trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to
itself.